Functional films composed of ferroelectric material or piezoelectric material are used in some electronic devices such as film capacitors, ferroelectric nonvolatile memories, film bulk acoustic wave resonators and the like. A film bulk acoustic wave resonator, for instance, is configured by sandwiching a functional film made of a piezoelectric material between an upper conductive film and a lower conductive film. The film bulk acoustic wave resonator operates as a high-frequency filter when a high-frequency signal is applied across the two conductors.
In order to achieve good device characteristics in such an electronic device, it is necessary to enhance the crystallinity of the functional film. In order to enhance it, it is important not only to optimize the functional film-forming method and conditions but also to improve the crystallinity of the lower conductive film serving as the base for the functional film. This is because the crystallinity of the lower conductive film serving as the base markedly affects the crystallinity of the functional film. A method of forming a lower conductive film with good crystallinity is taught by, for example, Japanese Patent Application Laid Open No. 11-312801.
The material of the lower conductive film needs to be one having high lattice matching and low reactivity with respect to the functional film formed thereon. When, for example, ZnO or some other such piezoelectric material of wurtzite crystal structure is used as the functional film material, therefore, the lower conductive film serving as the base is preferably a (111)-oriented film of a metal that assumes a face-centered cubic crystal structure, such as platinum (Pt), gold (Au), iridium (Ir) or rhodium (Rh).
However, the adhesion between the lower conductive film and the next layer thereunder serving as its base differs depending on the method and temperature used to form the lower conductive film. In some cases, only very weak adhesion can be obtained. In particular, the adhesion between a (111)-oriented film of a metal like platinum (Pt) that assumes a face-centered cubic crystal structure and the underlying insulator base markedly changes with the fabrication conditions. As a result, the lower conductive film may experience physical peeling that greatly degrades the reliability of the electronic device as a product. Moreover, when the fabricated electronic device is a film bulk acoustic wave resonator, slight exfoliation occurring at the interface between the lower conductive film and its base may markedly degrade the device characteristics. The lower conductive film is therefore required to have excellent adhesion.